Bacterial chemotaxis is one of a large number of sensory systems in bacteria that use two proteins, a histidine dependent protein kinase and the response regulator protein that is the kinase substrate. We propose to use modern nuclear magnetic resonance techniques to study the structures, dynamics and structural consequences of phosphorylation of three response regulators. These include: CheY, a member of the chemotaxis sensory transduction pathway in Escherichia coli; and SpoOF and SpoOA, members of the sporulation sensory transduction pathway in Bacillus subtilis. These three response regulators appear the have related distinct functions associated with the phosphorylation event and we hope to define the structural bases of these functional differences. We will also examine the structure of domains of the kinase CheA and the complex formed between CheA and CheY. The aim of this part of the proposal is to define the nature of the specific recognition between the kinase and its cognate response regulator(s).